Determining water quality parameters of aquifers in the Vientiane Basin, Laos, using geophysical and water chemistry data

The aim of this study is to test the possibility of using magnetic resonance sounding (MRS) and vertical electrical sounding (VES) together with groundwater chemistry data from shallow and deep wells to distinguish freshwater aquifers from salt affected groundwater and moreover determine water quality parameters directly from geophysical data. Three study areas within the Vientiane Basin, including 46 sites and 38 sampled wells, have been chosen for the study. The main water quality parameters affecting the water conductivity in the Vientiane Basin are total dissolved solids (TDS) and hardness and to a minor extent chloride, which all have shown to have fairly high correlation to the electrical conductivity ( EC ) of groundwater. The resistivity of the aquifers is highly variable but is most often above 4 Ωm, suggesting fresh water. No wells in the investigated areas have water quality parameters exceeding the limit for unacceptable drinking or irrigation water. There is a fairly strong relationship between the conductivity of water from deep wells and the conductivity of the main water bearing layer determined by VES, where the best fit is obtained using: EC [μS/cm] = 0.4694VES[μS/cm], R 2 = 0.69. However, due to equivalence in the VES interpretation and the difficulty to estimate the resistivity contribution from finer sediments, the equation may be seen and used as guidance to good water quality only within the Vientiane Basin area or in other areas with similar geology. The MRS results have been used to identify water layers in the VES inversion results to decrease the uncertainty in the interpretation The combination of MRS and VES techniques has been successful in distinguishing high conductive clay and mudstone from water bearing layers and moreover freshwater aquifers from salt affected water. Altogether, this makes MRS and VES very promising tools for guidance of future drilling and water quality estimation.

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